The present invention relates generally to optical inspection systems, and more specifically to optical inspection systems for obtaining data having very high signal-to-noise ratios.
The present invention relates generally to methods of inspecting patterned and unpatterned semiconductor wafers for defect acquisition and defect classification. One known method of inspecting wafers involves illuminating a relatively small spot on a wafer using a light source such as a laser beam. This small spot of light is scanned over the wafer using either raster or spiral scanning until the surface of the wafer to be inspected has been covered. The light scatters from the wafer structures into a hemisphere in various directions with intensities depending upon the structure on the wafer""s surface and any defects on the surface. The objective is to locate the defects on the wafer in the presence of scatter from both the defect and the intentional structures on the wafer""s surface associated with the device (e.g., computer processor or memory chip) being fabricated on the wafer. The scatter from the structures can be considered noise to the signal that is the scatter from the defect. The scattered light is commonly detected by a fixed number of detectors in fixed positions about the hemisphere.
In order to detect a defect, the signal-to-noise ratio of the collected scattered light must be sufficiently high. To obtain the most accurate defect analysis, it is desirable to locate the direction of the scattered light where the signal-to-noise ratio is the highest. The optimal region for the collection of scattered light can be anywhere on the hemisphere. By finding the optimum location in the hemisphere, the signal-to-noise ratios can easily be over 50 times greater than with fixed collection locations within the hemisphere. Therefore, to obtain the optimal signal to noise ratio, it is desirable to sample the entire hemisphere. Practically, it is not cost effective or easy to construct a complete hemisphere of detectors. Therefore, in current applications, a limited number of detectors are placed in what are hoped to be the optimal positions to obtain high signal-to-noise signals.
Unfortunately, in simulations of light scatter from relatively complicated structures with defects, the maximum signal-to-noise ratio location in the hemisphere can be considerably higher than signal-to-noise ratio from the fixed collector positions of current inspection systems. One option to compensate for the collector positions of the current inspection systems is to collect scattered light over a larger area of the hemisphere using larger detectors. This option may not be completely satisfactory since larger detectors collect more noise in addition to the increased signal collection. Thus, larger detectors do not necessarily improve the ratio of signal-to-noise. Another option is to use detectors that can be moved about the hemisphere to the locations of the highest signal-to-noise ratio. This option is not very viable, however, since the optimal location within the hemisphere changes as a function of the structure of the wafer and the types of defects, thereby requiring the frequent repositioning of the detectors. In light of the foregoing, it is desirable to have the ability to detect scattered light in a majority of the hemisphere of scattered light in order to obtain the highest possible signal-to-noise ratio for detecting defects as the wafer""s structure changes in the presence of a scanning spot over the wafer""s surface.
The present invention pertains to an optical inspection system capable of obtaining very high signal-to-noise ratio data and that is capable of high speed scanning rates so the defects are found quickly. The ability to obtain high signal-to-noise data is contributed to various aspects of the present invention. One aspect of the optical inspection system that contributes to a high signal-to-noise ratio is a set of lenses used to form an image of the inspected specimen at a Fourier plane. Another aspect of the optical inspection system is a substantially hemispherical shaped mirror system that provides a large collection numerical aperture that allows for the collection of substantially all of the hemisphere of scattered light from an inspected specimen. Several embodiments of the present invention also disclose techniques for enhancing the signal-to-noise ratio of image data received from the optical inspection system. The signals collected by the inspection system can help identify and classify the defect types according to the distribution of the light scattered from the wafer. This is possible since the angular distribution of light is representative of the defect types.
One aspect of the present invention pertains to an optical inspection system that includes a specimen to be analyzed, a light source, a set of optical lenses and a plurality of detectors. The light source transmits a light beam incident upon the surface of the specimen, which causes light rays to scatter from the surface of the specimen. The set of optical lenses is job positioned to receive the light rays scattering from the specimen and is configured to transmit the scattered light rays into a Fourier plane, whereby a map of the angular distribution of light scattered from the surface of the specimen is created at the Fourier plane. The plurality of detectors are placed in the Fourier plane to detect the scattered light rays, whereby the surface features of the specimen can be determined from the collected scattered light.
In an alternative embodiment of the inspection system, a substantially hemispherical mirror is placed over an area of the specimen to be inspected, the mirror being configured to collected and then direct the scattered light from the specimen towards the set of optical lenses. The mirror is capable of collecting and directing light rays that scatter into substantially a full hemisphere from the surface of the specimen.
Another aspect of the present invention relates to a method for detecting defects on a specimen using the optical inspection system. The method includes irradiating a spot on the specimen with a light source, the light source causing light rays to scatter from the surface of the specimen. Then detecting the image of the scattered light rays at a Fourier plane, the Fourier plane created by a set of Fourier plane forming lenses within the optical inspection system. Then multiplying the signal value for each discrete portion of the irradiated spot by a respective vector value to obtain a respective adjusted signal value. The respective vector values cause the respective adjusted signal value to increase if the respective signal value is associated with a high defect signal. The respective vector values also cause the respective adjusted signal value to decrease if the respective signal value is associated with a noise defect signal. And then, evaluating a sum total of the respective adjusted signal values for the irradiated spot to determine whether a defect exists within the irradiated spot on the specimen.